One problem encountered when handling small fluid volumes is to effectively mix different fluids together. For instance, poor mixing can occur in droplet-based microfluidic devices, where the fluids are not confined in channels. In droplet based systems, small droplets of fluid (e.g., fluid volumes of about 100 microliters or less) are moved and mixed together on a surface. In some cases, it is desirable to add a small volume of a reactant to a sample droplet to facilitate the analysis of the sample, without substantially diluting it. In such cases, there is limited ability to mix the two fluids together because there is no movement of the fluids to facilitate mixing.
Embodiments of the present invention overcome these problems by providing a device and method that facilitates the movement and mixing of small volumes of fluids.